Cystic fibrosis (CF) is the most common lethal inherited disease in Caucasians in North America. Among the bacterial species that cause the chronic respiratory tract infections that characterize CF are members of the Burkholderia cepacia complex (Bcc). Infection with Bcc is associated with significantly increased rates of morbidity and mortality in CF, and is generally considered an absolute contraindication to lung transplantation. Species within the Bcc are both phylogenetically very closely related and phenotypcially nearly indistinguishable. Despite this close relationship, however, Bcc species show a striking difference in the frequency with which they cause infection in CF, with B. cenocepacia being particularly common in this patient population. Furthermore, so-called epidemic B. cenocepacia strains infect a great many patients, and appear to have an enhanced capacity for inter-patient transmission and virulence. At present, the biologic underpinnings of these striking differences in disease frequency and virulence are unknown. We hypothesize that specific bacterial factors contribute to the disproportionate distribution and virulence of epidemic B. cenocepacia strains in CF. In this Exploratory/Developmental Project (R21), we will test this hypothesis by using a comparative genomics approach to identify genes that are specifically associated with epidemic B. cenocepacia. We will take advantage of our extensive strain collection, the large number of recently sequenced Burkholderia strains, innovative microarray technology, and 20 years of epidemiologic analysis of Bcc to identify these genetic elements. Project Narrative--Bacteria within the Burkholderia cepacia complex cause severe, life threatening respiratory infection in persons with cystic fibrosis (CF). These bacteria are typically resistant to most available antibiotics, making effective treatment of infection impossible. Furthermore, infection may spread between patients, and this has resulted in strict policies to separate CF patients from each other. Certain specific strains are more commonly found to cause infection and are associated with worse disease. The primary aim of this project is to discover the specific factors that allow these strains to spread and cause severe infection in persons with CF.